not sure, i have a hang on the back prism skim and it collects alot of salt on the edges and sometimes when they get big enought the fall into the tank. may it fell on it and didn't have enough flow to get it off?? or i had a coral beauty that as a result of the decay removed and brought in to PC for credit, maybe it was picking at it?? hard to tell, i came home once and my Naso was pulling at it like it was a piece of nori?? i'm suprised it came back, i've just been doin water changes every week for the past two months now and i even have luck with a couple small acros under the pc's